HandleHLSSegment mapped every non-ErrSegmentNotFound error from the
segment-retrieval/recovery path to a generic 500 "Failed to load segment".
When a transcode process starts and then exits non-zero, WaitForSegment
returns a wrapped playback.ErrTranscodeFailed, which fell through to that
500 — observed in production as repeated 500s on seg_00000.ts that drove an
8x client retry storm and crash-log uploads.
The segment will never materialize once its transcode has died, so this is a
not-found condition: Jellyfin's DynamicHls handler falls through to a
PhysicalFileResult for the absent file, which ASP.NET serves as 404, never
500. Map ErrTranscodeFailed to 404 alongside ErrSegmentNotFound via a small
extracted hlsSegmentErrorResponse helper, reserving 500 for genuinely
unexpected errors. Adds a unit test pinning the mapping.
